AbstractThe monarch butterfly,Danaus plexippus(Linnaeus 1758) is a charismatic butterfly known for its multi-generational migration in Eastern North America, and is emerging as an important system in the study of evolutionary genetics. Assigning genes or genetic elements to chromosomes is an important step when considering how genomes evolved. The monarch butterfly has a reported haploid chromosome number of 30. The specimens that this count is based on are from Madras, India whereD. plexippusdoes not occur. As a consequence the reported haplotype for theD. plexippusis incorrect. A literature review revealed that the specimens this count was based on were most likelyDanaus genutia(Cramer 1779), which Linnaeus and others conflated withD. plexippus. In a 1954 Opinion (#282) the International Code of Zoological Nomenclature decided that the nameD. genutiarefers to specimens originating from the Orient andD. plexippusto those from North America. To clarify the haplotype number ofD. plexippusfrom North America I conducted chromosome squashes of male 5th instar larvae. All specimens examined had a haploid chromosome count of n = 28. Following these new data, I then reconstructed the evolution of haplotypes throughout the Danainae subfamily to provide a testable hypothesis on the evolution of chromosome number for this group.